Vol. 23 (December 2020) Insect Environment Research Articles Oxygen consumption by Ancylostomia stercorea Zeller (Lepidoptera: Pyralidae) larvae: effect of temperature and body weight Stanford A. Lawrence and Ayub Khan Department of Life Sciences University of the West Indies, St. Augustine, Trinidad Corresponding author: [email protected] Abstract The influence of temperature on the respiration rate of Ancylostomia stercorea larvae was investigated because of modest research on this serious pigeon pea pest in the region. The respiratory rates of 3rd, 4th and 5th instar larvae were assessed at four temperatures 20, 25, 30 and 0 0 35 C. The mean oxygen consumption was greatest at 25 C for all three instars. The highest Q10 values were observed for all three instars at the temperature range 20-300C. There was a negative scaling relationship between oxygen consumption and A. stercorea larval body weight. Keywords: Pigeonpea, Ancylostomia stercorea, oxygen consumption, pod borer Introduction Pigeonpea (Cajanus cajan) (L) is an important drought tolerant grain legume crop grown in developing countries. The crop is ranked sixth in the world in terms of grain legume production with 5.96 x 106 tonnes produced in 2018 (FAOSTAT 2020). The major pigeon pea producing countries are India, Burma, Uganda and the Dominican Republic. In the Caribbean region the legume is grown mainly under rain fed conditions by small farmers, to whom the crop represents an important source of income. FAOSTAT (2020) ranked Trinidad, Jamaica and Grenada as 16th, 17th and 18th respectively in terms of pigeon pea production worldwide. The pigeon-pea crop is subject to insect attack throughout its growth; however, the most serious economic pests are those which attack the reproductive structures, including flowers, pods and seeds (Shanower et al., 1999; Buckmire 1978). Buckmire (1978) identified a number of pod borers that affect the crop in the Caribbean region. These include Acylostomia strecorea Zeller (Lepidoptera:Pyralidae), Heliothis virescens F. (Lepidoptera:Noctuidae), Chalcodermus angulicollis Fahraeus (Coleoptera:Cuculionidae) and Fundella pellucens Zeller 8 Vol. 23 (December 2020) Insect Environment (Lepidoptera:Pyralidae). Ancylostomia stercorea was identified as the most serious pod borer throughout the region with seed damage in the unprotected crop being as high as 60% in some countries. This pest is a multivoltine species (Bennett 1960). Oval shaped eggs, 0.6 mm in length and 0.4 mm in width are oviposited on young C. cajan pods. There are five larval instars and developing larvae feed on seeds inside pods and later pupate in the soil. The life cycle is completed in 26-32 days (Khan et al., 2003). Insects inside pods would be exposed to a range of temperatures and this would affect their respiratory rate and metabolism (Lal et al., 2005; Rosenzweig and Hillel, 1998). Petzoldt and Seaman (2005) noted that climate change resulting in increased temperature will impact the insect population in several ways including increased population size, metabolic and rate food consumption. The current study aimed at evaluating the influence of temperature on the respiration rate and scaling relationships between larval body weight and oxygen consumption of A. stercorea 3rd, 4th and 5th instar larvae at different temperatures. Materials and Methods Third, fourth and fifth instar larvae of A. stercorea were obtained the University Field Station Valsayn Trinidad and kept in pigeon pea pods, in well ventilated plastic containers (15cm width x 20cm length x 10cm depth) with organza covered mesh (250 mesh/cm2) at the top. Respiration rate was measured using a microrespirometer based on the design by Lee (1995) (Figure 1) which was placed in a constant temperature Thermo Fisher Scientific Precision Controlled 280® series water bath (Model 2849). A mercury thermometer (-20 to 1100C) was used to ensure that the desired water temperature was achieved. The live larval weight (mg) was taken at the end of each experiment. There were five replicates for each instar and oxygen consumption was assessed at 10-minute intervals over 90 minutes at four temperatures 20ºC, 25ºC, 30ºC and 35ºC. Q10 values indicative of temperature sensitivity of metabolic rate were 10 calculated using the following equation: Q10= (R2/R1) / (T2-T1), where: R1 and R2 are the 0 respiration rates at temperatures T1 and T2 in C. Q10 represents the factor by which the metabolic 0 rate increases with a 10 C change in temperature. Values of Q10 > 1 indicate an increase in oxygen consumption (metabolic rate) after exposure to increased temperature, while values of Q10 < 1 indicate a decrease in oxygen consumption after exposure to increased temperature. 9 Vol. 23 (December 2020) Insect Environment Scaling relationships were determined for combined 3rd, 4th and 5th instar larvae at each of four temperatures tested. These relationships were in the form: R = aWb where: R = oxygen consumption rate, W = bodyweight of larva, a = constant (oxygen consumption at weight 0 = y- intercept) and b = slope of graph of log oxygen consumption vs. log weight. Fig 1: Microrespirometer design (after Lee 1995) Results Mean oxygen consumption for 3rd – 5th instar A. stercorea larvae was significantly higher (P<0.001) at 250C for all instars compared to the other temperatures tested. However, oxygen consumption at 20, 30 and 350C was not significantly different (P>0.001) from each other for each instar examined (Table 1). The 3rd – 5th instar larvae of A. stercorea exhibited highest oxygen consumption at 250C compared to the other temperatures tested. Oxygen consumption 0 declined rapidly at temperatures higher than 25 C for all three larval instars (Table 1). Q10 values were higher at temperatures between 200C and 300C compared to 25 and 350C (Table 2). 10 Vol. 23 (December 2020) Insect Environment There was a general decrease in oxygen consumption as larval body weight increased as indicated by the negative values of the slope (b) at all temperatures. Comparison of the slopes indicated that they were significantly different from zero (P<0.05) however, there was no significant difference (P>0.05) between them at any of the temperatures tested (Table 3). Discussion The results from the current study indicated that the respiration rate of A. stercorea larvae fluctuated with temperature and that maximum oxygen consumption occurred at 250C for all three instars. The highest oxygen consumption rate was observed at 250C for 5th instar larvae of approximately 1.4µl/mg/h. However, the respiration rate declined steadily at 300C and 350C. Lee (1995) noted that invertebrates’ oxygen consumption is often directly related to temperature over the range of 0-250C. He also observed a 40% decrease in respiration rate, using the same microrespirometer technique, to assess the acclimation of diapausing adults of the convergent lady beetle Hippodamia convergens Guerin-Meneville (Coleoptera:Coccinellidae) to warmer temperature and holds true for A. stercorea 3rd, 4th and 5th instar larvae. Similarly, Bennett et al., (1998) found that lepidopteran larvae could increase respiration rate rapidly with increasing temperatures in the low to mid-temperature ranges (0-200C) but at sustained temperatures 300C and above, larval respiration rate appeared to level off or decrease and can be physiologically stressful. Similar respiration patterns were observed at 300C and 350C for the three instars of A. stercorea. In insects there are three periods of respiratory gas exchange referred to as when the spiracles are open, fluttering and closed (Nation 2008, Sláma 1988, Sláma and Karel 1999). He described this process as discontinuous gas exchange and which was evident in A. stercorea larvae examined which rapidly increased/decreased respiration rate over a range of time and temperatures and is associated with specific changes in the mechanical pressure within the tracheal system. o The higher Q10 values observed at lower temperatures (20-30 C) may be interpreted as an adaptation of A. stercorea that reduces energetic loss at lower temperatures. Studies on the high Arctic wooly bear caterpillar Gynaephora groenlendica (Homeyer) (Lepidoptera: Lymantriidae) indicated that Q10 value decreased with increasing temperature for both fed and starved larvae 11 Vol. 23 (December 2020) Insect Environment (Bennett et al., 1998). Similar results were obtained for larvae of A. stercorea in the current study. The scaling relationship between oxygen consumption and body weight clearly showed that A. stercorea larvae oxygen consumption decreased as larval body weight increased, similar to that of Clarke (1957) studying Locusta migratoria L. where a high correlation between the trend lines for weight and oxygen consumption in the early instars was observed. Global temperatures are expected to rise as a result of climate change and have been predicted to adversely affect agricultural crops. Pigeonpea, the host plant of A. stercorea can be grown on marginal land with minimal input and has been touted as one of the crops able to withstand the effects of climate change as it has been shown, among other characteristics, to be drought and heat tolerant. This study demonstrated that oxygen consumption by A. stercorea larvae was temperature sensitive. While small temperature changes did not negatively affect larval oxygen consumption, elevated temperatures expected as a result of global warming may thus adversely affect this pest despite the favorable outlook for its host plant. References Bennett, F.D 1960. Parasites of Ancylostomia stercorea (Zell.) (Pyralidae, Lepidoptera) A pod borer attacking pigeon pea in Trinidad. Bulletin of Entomological Research 50: 737-757. Bennett, V. A., Kukal, O. and Lee Jr, R.E. 1998. Metabolic opportunists: Feeding and temperature influence the rate and pattern of respiration in the high Arctic wollybear caterpillar, Gynaephora groenlandica (Lepidoptera: Lymantriidae). Journal of Experimental Biology (202): 47-53. Buckmire, K.U. 1978. Pests of grain legumes and their control in the Commonwealth Caribbean. In: Pests of Grain Legumes: Ecology and Control. H. F. Van Emden and T. Ajibola Taylor (Eds.): pp. 179 - 184 Academic Press, New York.